Aluminium composite materials consisting of at least one aluminium core alloy and at least one aluminium brazing layer arranged on one or both sides of the aluminium core alloy are used for producing brazed constructions. Often the brazed constructions have a plurality of brazing points, as is the case for example with heat exchangers. For this purpose various brazing methods are used for brazed metal structural parts. One of the most common methods is the so-called “controlled atmosphere brazing (CAB) method”, in which the aluminium structural parts are as a rule brazed using fluxes and during the brazing process are exposed to an inert gas atmosphere, for example a nitrogen atmosphere. Other thermal joining methods also use fluxes and soften the aluminium brazing material also in the presence of a protective gas. The use of corrosive or non-corrosive fluxes has disadvantages however, for example increased plant costs and technical problems concerning the interaction of residues of the flux with for example additions of coolant in a heat exchanger. Furthermore the use of fluxes is also problematic as regards the avoidance of environmental impact and from work safety aspects. Finally, in the CAB method the use of Mg-containing core alloys is problematic, since magnesium adversely affects the brazing properties under an inert gas atmosphere. In addition, the brazed structural parts can also be affected by discolourations. From international patent application WO 2010/000666 A1, a method is furthermore known for flux-free brazing with the CAB method, in which the aluminium brazing layer consists of a first aluminium brazing layer and a second aluminium brazing layer. The second aluminium brazing layer consists of an Al—Si aluminium alloy that contains in addition to 5 wt. %-20 wt. % silicon also 0.01 wt. %-3 wt. % magnesium. The first aluminium brazing alloy contains on the other hand 2 wt. %-14 wt. % silicon and less than 0.4 wt. % magnesium. The two-layer structure of the aluminium brazing layer is however unsatisfactory since the production of the two-layer aluminium brazing layer involves higher costs.
Moreover a significant disadvantage of current two-layer structures, for example with an outer-lying cladding of pure aluminium, may be that its use is not compatible with fluxes. Unsatisfactory brazing results, for example on account of a temporarily worse furnace atmosphere with too high an oxygen partial pressure or too high a moisture content in the atmosphere, cannot be compensated for example by the use of fluxes.
The second method, which is often employed, is vacuum brazing, in which the structural parts to be brazed are brazed in an atmosphere at a very low pressure, for example about 10−5 mbar or less. Vacuum brazing can be carried out without fluxes, though usually a certain amount of magnesium is added to the aluminium brazing material in order to obtain a better brazing result. The use of Mg-containing brazing material is associated with additional negative effects, for example the need for frequent furnace cleaning routines. Vacuum brazing is moreover very complicated in terms of apparatus requirements and therefore very cost-intensive. From the Japanese publications JP 04-1000696, JP 04-100674 as well as JP 05-154693, the use of an alkaline-pickled aluminium composite material in a vacuum brazing method or with fluxes in a CAB method is known.
The specification U.S. Pat. No. 5,102,033 describes on the other hand a method in which an aluminium composite material consisting of an aluminium core alloy and an aluminium brazing alloy layer is pickled with an acidic pickling solution that contains a mixture of nitric acid and hydrofluoric acid and is then brazed by vacuum brazing. Although the US specification also mentions conventional brazing methods, these are however as a rule characterised by the use of fluxes, in so far as they are not carried out in a vacuum. Also, an aluminium brazing alloy for brazing using fluxes is disclosed in WO 98/45082.
The published application WO 2013164466 A1 of the Applicant in fact discloses the principle of the use of an acid-pickled aluminium composite material in a flux-free thermal joining method. Further details are, however, not disclosed.
In U.S. Pat. No. 3,779,839A, a method is also disclosed in which aluminium structural parts are pickled in an alkaline or acid solution and are then joined by flux-free brazing.
JP H11-285817 A relates to an aluminium brazing alloy with a specified Mg, Bi and Be content that is coated with a forming oil after an acidic pickling. After removing the forming oil the alloy can then be brazed flux-free in an inert gas atmosphere.
Against this background the object of the present invention is to propose the use of an aluminium composite material with an aluminium brazing layer in a thermal joining method, which leads to a further reduction in costs and a lower environmental impact. In addition a method for producing the aluminium composite material, a method for thermal joining as well as a thermally joined construction should also be provided.